I'm pretty excited.  I think I found my first hammer today.

Yep! You got me all excited!
That's the real deal. Full-grooved axe/hammer. Hard to believe that ever had a wood chopping/cutting bit end on it as rounded as it is now. Maybe it didn't. Maybe they just had a lot of things that needed to be hammered!? But, because of that groove, that thing was hafted to a handle at one time. And the full grooved were the first or earliest type. I'd guess that artifact to be 5-6000 years old or older.

I'm sure you remember Exactly where you found that at, so now go back out there and act like a goat on a rope. Work your way out in circles and look real close at every rock!
Nice Find!

Well I figured that's where you've been, but now you're just pulling my leg, dude!
I mean look at that rock. Big ol'black and white coarse grains that you know are going to be falling out all the time. And that seam of white quartz feldspar mixture would obviously crack and break apart! Why would they ever go to the trouble of making a grooved axe/hammer out of rock like that?
Regardless, sure looks like another grooved stone artifact to me.
That's the great thing about this hobby. Every time you find something there are a dozen questions. And when you find the answers to the first dozen there a dozen more new ones you never thought of.

Why are you finding the same tool type in this area?
And what was it used for?
And no other artifacts?

The stone in this one looks like a granite or pegmatite granite and you should be able to note a source location or two for this stone somewhat locally.

"No, I don't collect arrowheads. I just collect full-grooved axe/hammer tools, thank you."
Jeesh!

I was wondering some of those same things.  I don't see the practicality of using this rock as a hammer. There are better options around here that would be more durable.
As far as a source for this rock, it could be a thousand miles away. All our rocks were deposited here far from their source a long time ago. We have a crazy mix of soils, sand, gravel, and rocks anywhere from the size of pebbles up to the size of cars. No exposed bedrock anywhere around here though.
There is another artifact I'm going to try to get tommorow. I will at least get a picture of it. Our ground is frozen and I didn't have the proper tools to dislodge this next rock. I'm not sure what it is. I will just have to leave you in suspense until then.

Hey farmdude, that snow sprinkled in the grass sure looks pretty,... and cold!
Ya kno, you can't hunt rocks if the ground is covered with 3 feet of snow.
Hope the harvest is in and went well.

I hope you got an empty barn up there somewhere for all these rocks you're going to be bringing home! You're going to have to open your own museum! I don't don't know what that rock is but it belongs in a museum! Maybe your museum?
I think it's probably limestone and it's upsidedown in the pictures. I'm guessing it's a sedimentary layer of limestone that settled into and filled a hole on another surface, or the top of the layer below the limestone.
It could be a fossilized dinosaur vertebrae or an extinct rare coral formation, if you want to think of all the possibilities.
I can't see from a picture whether that piece has been worked and ground. You did find it in an area with other ground objects? And nature may have started the piece and man just altered it a little?

I could find precious little about the history of finds of fullgrooved axe heads or tools from South Dakota, but will continue looking. Iowa has a great archaeology department.
I'm putting a link to a site talking about the curation and classifying of artifacts from South Dakota. I don't expect you to absorb all the information in one read, but surf thru it and read what you want. Mainly the article gives you a good look at how archaeologist operate. And take special note of the Knife River Flint "flakes" from reduction in the stone tool making process mentioned.
https://nmnh.typepad.com/rogers_archaeology_lab/collections-management/

Totally possible about the old collection, farmdude. Wouldn't be the first time that's happened.

Yeah, check that link out. I know some of it will be of interest to you and the rest gives you a good look at the archaeologists point of view. HA!

Stay warm up there!

If you ever make it up to the Dakotas, give me a shout NCheadhunter!